regard continuous colons as part of a word

Question

I want to regard continuous colons as part of a word. How can I configure the iskeyword option?

For example,

• in ::hoge, the whole thing is a word
• in :hoge, : is a word, and hoge is also a word
• in fuga:hoge, fuga is a word, : is also a word, and hoge is also a word
• in fuga::hoge, the whole thing is a word
• in hoge:::, the whole thing is a word